3,186 research outputs found
Locally critical point in an anisotropic Kondo lattice
We report the first numerical identification of a locally quantum critical
point, at which the criticality of the local Kondo physics is embedded in that
associated with a magnetic ordering. We are able to numerically access the
quantum critical behavior by focusing on a Kondo-lattice model with Ising
anisotropy. We also establish that the critical exponent for the q-dependent
dynamical spin susceptibility is fractional and compares well with the
experimental value for heavy fermions.Comment: 4 pages, 3 figures; published versio
Numerical evidence for the spin-Peierls state in the frustrated quantum antiferromagnet
We study the spin- Heisenberg antiferromagnet with an
antiferromagnetic (third nearest neighbor) interaction on a square
lattice. We numerically diagonalize this ``-'' model on clusters up
to 32-sites and search for novel ground state properties as the frustration
parameter changes. For ``larger'' we find enhancement of
incommensurate spin order, in agreement with spin-wave, large- expansions,
and other predictions. But for intermediate , the low lying excitation
energy spectrum suggests that this incommensurate order is short-range. In the
same region, the first excited state has the symmetries of the columnar dimer
(spin-Peierls) state. The columnar dimer order parameter suggests the presence
of long-range columnar dimer order. Hence, this spin-Peierls state is the best
candidate for the ground state of the - model in an intermediate
region.Comment: RevTeX file with five postscript figures uuencode
Quantum critical transport, duality, and M-theory
We consider charge transport properties of 2+1 dimensional conformal field
theories at non-zero temperature. For theories with only Abelian U(1) charges,
we describe the action of particle-vortex duality on the
hydrodynamic-to-collisionless crossover function: this leads to powerful
functional constraints for self-dual theories. For the n=8 supersymmetric,
SU(N) Yang-Mills theory at the conformal fixed point, exact
hydrodynamic-to-collisionless crossover functions of the SO(8) R-currents can
be obtained in the large N limit by applying the AdS/CFT correspondence to
M-theory. In the gravity theory, fluctuating currents are mapped to fluctuating
gauge fields in the background of a black hole in 3+1 dimensional anti-de
Sitter space. The electromagnetic self-duality of the 3+1 dimensional theory
implies that the correlators of the R-currents obey a functional constraint
similar to that found from particle-vortex duality in 2+1 dimensional Abelian
theories. Thus the 2+1 dimensional, superconformal Yang Mills theory obeys a
"holographic self duality" in the large N limit, and perhaps more generally.Comment: 35 pages, 4 figures; (v2) New appendix on CFT2, corrected
normalization of gauge field action, added ref
APOE genotype and entorhinal cortex volume in non-demented community-dwelling adults in midlife and early old age
Copyright © 2012 IOS PressThis article has been made available through the Brunel Open Access Publishing Fund.The apolipoprotein E (APOE) ε4 allele is a risk factor for the neuropathological decline accompanying Alzheimer's disease (AD) while, conversely, the ε2 allele offers protection. One of the brain structures exhibiting the earliest changes associated with the disease is the entorhinal cortex. We therefore investigated the volumes of the entorhinal cortex and other structures in the medial temporal lobe including the parahippocampal gyrus, temporal pole, and inferior, middle, and superior temporal cortices, in relation to APOE genotype. Our main objectives were to determine if (a) volumes systematically varied according to allele in a stepwise fashion, ε2 > ε3 > ε4, and (b) associations varied according to age. We investigate this association in 627 non-demented community-dwelling adults in middle age (44 to 48 years; n = 314) and older age (64 to 68 years; n = 313) who underwent structural MRI scans. We found no evidence of APOE-related variation in brain volumes in the age groups examined. We conclude that if a ε2 > ε3 > ε4 pattern in brain volumes does emerge in non-demented adults living in the community in old age, it is not until after the age of 68 years.This study was funded by the UK Leverhulme
Trust, the British Academy, the NHMRC
Research Fellowship No. 471501, the NHMRC Research Fellowship No.#1002560, the National Health and Medical Research Council of Australia Unit Grant No. 973302, Program Grant No. 179805, Project grant No. 157125; Program grant no. 350833, and the National Computational Infrastructure. This article is made available through the Brunel Open Access Publishing Fund
Entangling strings of neutral atoms in 1D atomic pipeline structures
We study a string of neutral atoms with nearest neighbor interaction in a 1D
beam splitter configuration, where the longitudinal motion is controlled by a
moving optical lattice potential. The dynamics of the atoms crossing the beam
splitter maps to a 1D spin model with controllable time dependent parameters,
which allows the creation of maximally entangled states of atoms by crossing a
quantum phase transition. Furthermore, we show that this system realizes
protected quantum memory, and we discuss the implementation of one- and
two-qubit gates in this setup.Comment: 4 pages, REVTEX, revised version: improvements in introduction and
figure
Magnetic properties of strongly disordered electronic systems
We present a unified, global perspective on the magnetic properties of
strongly disordered electronic systems, with special emphasis on the case where
the ground state is metallic. We review the arguments for the instability of
the disordered Fermi liquid state towards the formation of local magnetic
moments, and argue that their singular low temperature thermodynamics are the
``quantum Griffiths'' precursors of the quantum phase transition to a metallic
spin glass; the local moment formation is therefore not directly related to the
metal-insulator transition. We also review the the mean-field theory of the
disordered Fermi liquid to metallic spin glass transition and describe the
separate regime of ``non-Fermi liquid'' behavior at higher temperatures near
the quantum critical point. The relationship to experimental results on doped
semiconductors and heavy-fermion compounds is noted.Comment: 25 pages; Contribution to the Royal Society Discussion Meeting on
"The Metal-Non Metal Transition in Macroscopic and Microscopic Systems",
March 5-6, 199
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White matter hyperintensities and within-person variability in community-dwelling adults aged 60–64 years
Estimates of white matter hyperintensities (WMH) derived from T2-weighted MRI were investigated in relation to cognitive performance in 469 healthy community-dwelling adults aged 60–64 years. Frontal lobe WMH but not WMH from other brain regions (temporal, parietal, and occipital lobes, anterior and posterior horn, periventricular body) were associated with elevated within-person reaction time (RT) variability (trial to trial fluctuations in RT performance) but not performance on several other cognitive tasks including psychomotor speed, memory, and global cognition. The findings are consistent with the view that elevated within-person variability is related to neurobiological disturbance, and that attentional mechanisms supported by the frontal cortex play a key role in this type of variability
Universal low-temperature tricritical point in metallic ferromagnets and ferrimagnets
An earlier theory of the quantum phase transition in metallic ferromagnets is
revisited and generalized in three ways. It is shown that the mechanism that
leads to a fluctuation-induced first-order transition in metallic ferromagnets
with a low Curie temperature is valid, (1) irrespective of whether the magnetic
moments are supplied by the conduction electrons or by electrons in another
band, (2) for ferromagnets in the XY and Ising universality classes as well as
for Heisenberg ferromagnets, and (3) for ferrimagnets as well as for
ferromagnets. This vastly expands the class of materials for which a
first-order transition at low temperatures is expected, and it explains why
strongly anisotropic ferromagnets, such as UGe2, display a first-order
transition as well as Heisenberg magnets.Comment: 11pp, 2 fig
Columnar Fluctuations as a Source of Non-Fermi-Liquid Behavior in Weak Metallic Magnets
It is shown that columnar fluctuations, in conjunction with weak quenched
disorder, lead to a T^{3/2} temperature dependence of the electrical
resistivity. This is proposed as an explanation of the observed
non-Fermi-liquid behavior in the helimagnet MnSi, with one possible realization
of the columnar fluctuations provided by skyrmion lines that have independently
been proposed to be present in this material.Comment: 4pp, 4 figure
Sign change of the Grueneisen parameter and magnetocaloric effect near quantum critical points
We consider the Grueneisen parameter and the magnetocaloric effect near a
pressure and magnetic field controlled quantum critical point, respectively.
Generically, the Grueneisen parameter (and the thermal expansion) displays a
characteristic sign change close to the quantum-critical point signaling an
accumulation of entropy. If the quantum critical point is the endpoint of a
line of finite temperature phase transitions, T_c \propto (p_c-p)^Psi, then we
obtain for p<p_c, (1) a characteristic increase \Gamma \sim T^{-1/(\nu z)} of
the Grueneisen parameter Gamma for T>T_c, (2) a sign change in the Ginzburg
regime of the classical transition, (3) possibly a peak at T_c, (4) a second
increase Gamma \sim -T^{-1/(nu z)} below T_c for systems above the upper
critical dimension and (5) a saturation of Gamma \propto 1/(p_c-p). We argue
that due to the characteristic divergencies and sign changes the thermal
expansion, the Grueneisen parameter and magnetocaloric effect are excellent
tools to detect and identify putative quantum critical points.Comment: 10 pages, 7 figures; final version, only minor change
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